Phentolamine mesylate is a competitive nonselective antagonist of α-adrenergic receptors. In fact, it binds α1 and α2 receptors with similar affinities. Therefore, its main effects are hypotension and tachycardia.

Property

Phentolamine mesylate is a white or off-white, odorless crystalline powder. Its solutions are acid to litmus. It is freely soluble in water and in alcohol, and slightly soluble in chloroform. It melts at about 178°C.

Uses

Phentolamine mesylate is an alpha-adrenergic blocker used to treat hypertensive episodes, diagnose pheochromocytoma, treat norepinephrine administration site reactions, and reverse soft tissue anesthesia and mydriasis.

Mechanism of action

Phentolamine mesylate is a reversible, competitive antagonist at alpha-1 and alpha-2 adrenergic receptors.2,5 It causes vasodilation of vascular smooth muscles. Pupil dilation is primarily controlled by the radial iris dilator muscles surrounding the pupil, which are activated by the alpha-1 adrenergic receptors. Phentolamine reversibly binds to these receptors and reduces pupil diameter. By blocking alpha-1 adrenergic receptors, phentolamine can also be used to reverse mydriasis induced by muscarinic antagonists.1

Side effects

Common side effects of phentolamine mesylate include:

acute and prolonged hypotensive episodes

fast heart rate

cardiac arrhythmias

weakness

dizziness

flushing

dizziness upon standing (orthostatic hypotension)

stuffy nose

nausea

vomiting

diarrhea

Acute Toxicity

No deaths due to acute poisoning with phentolamine have been reported.

Oral LD50's (mg/kg): mice, 1000; rats, 1250.

Contraindications

Myocardial infarction, history of myocardial infarction, coronary insufficiency, angina, or other evidence suggestive of coronary artery disease; hypersensitivity to phentolamine or related compounds.

Reference

[1] Raja SN, Treede RD, Davis KD, Campbell JN: Systemic alpha-adrenergic blockade with phentolamine: a diagnostic test for sympathetically maintained pain. Anesthesiology. 1991 Apr;74(4):691-8. doi: 10.1097/00000542-199104000-00012.